The largest study to examine the risks of hormone-based birth control has concluded the contraceptives carry a small risk of stroke and heart attack, depending on the method and type of hormone used. But the risk for individual women remains extremely low, particularly in younger women. Danish researchers wrote in the New England Journal of Medicine that the findings suggest a higher risk of stroke in particular for women using vaginal rings, and possibly hormonal skin patches -- though the second finding was based on a smaller group of women and could have been due to chance.